1. Field of the Invention
This invention relates to new and useful improvements in methods for measuring fracture toughness of extremely hard and brittle materials such as tungsten carbide, and especially sintered tungsten carbide.
2. Discussion of the Prior Art
In the 1972 Ph.D. thesis by R. C. Lueth at Michigan State University there was reported a fracture toughness measurement procedure carried out on tungsten carbide. The Lueth procedure involved the use of long, slender specimens which are not readily available commercially. In the Lueth procedure a crack was introduced into a specimen and and the propagation of the crack arrested. Then the propagation of the crack further was measured and fracture toughness calculated. The main difficulty with the Lueth procedure was that the initial crack was hard to control and often ran most of the way through the sample.
In "FRACTURE TOUGHNESS OF CEMENTED TUNGSTEN CARBIDES" by Ingelstrom and Nordberg, Engineering Fracture Mechanics, Vol. 6, p. 597 (1974), another procedure is described for toughness testing. The authors used single-edge notched bend test specimens. A diamond indentation method of crack initiation was used. The same authors also used a compact tension specimen configuration in which they initiated the crack by an impact method and produced the required initial crack arrest by a strong lateral confining stress. The residual effect of the confining stress was shown to affect the measured fracture toughness if too large, yet did not produce the necessary crack arrest if too small.
An impact fracture toughness method is also described in Shockey and Curran U.S. Pat. No. 3,793,874. This procedure involves the formation of internal cracks in a specimen under repeated impacts and determination of crack size distribution. The change in crack distribution is used to calculate a.sub.c, the critical crack half length of radius above which cracks grow and below which they do not grow.